1. Field of the Invention
This invention relates to automatic focusing devices and, more concretely, to automatic focusing devices for camera having image pickup means, such as video cameras.
2. Description of the Related Art
Conventionally the automatic focusing device of the video camera has employed the so-called "hill climb method" as was derived by noticing that the high-frequency component in the video signal corresponds to the degree of fineness of the shot picture. To control focusing, the focus adjusting ring (hereinafter referred to as the "helicoid") of the photographic optical system is rotated so that that high-frequency component becomes maximum. This method is described in detail in "NHK Technology Research Report" in p. 21, vol. 17, No. 1,1965, serial No. 86, entitled "Automatic Focus Adjustment of the Television Camera by the Hill Climb Servo Mehod" by Ishida et al.
FIG. 1 in block diagram shows the fundamental structure of the automatic focusing device based on this hill climb method. A lens 10 serving as photographic optical system forms an image on a focal plane which is converted to an electrical video signal by a camera circuit 12. As the video signal produced from the camera circuit 12 includes a high-frequency component, a high-pass filter (HPF) 14 extracts it. The high-frequency component from the HPF 14 is detected by a detector 16. The output level of the detector 16 is proportional to the amount of the high-frequency component, representing the degree of fineness of the image of an object to be photographed. In relation to the adjusted position of the helicoid, the output level of the detector 16 has a characteristic of upward convexity as shown in FIG. 2 with the center at the focusing position A of the sharpest focus. A difference-hold circuit 18 samples and holds the output of the detector 16 (hereinafter referred to as the "focus voltage") in predetermined discrete times, and produces an output signal corresponding to the amount of temporal variation of the focus voltage. That is, the output signal of the difference-hold circuit 18, as shown by a characteristic curve in the lower half of FIG. 2, takes a zero point at the focusing position A. On its front and rear sides, the polarity of the signal inverts, taking the opposite signs to each other, or the positive and negative ones respectively. It should be noted that the characteristics shown in FIG. 2 remain the same regardless of whether the focusing of the lens is performed from the closest object distance to infinity, or vice versa.
A motor drive circuit 20 determines which direction in which an electric motor 22 for driving the lens 10 (more specifically, its helicoid) is to be rotated depending on the polarity of the output of the difference-hold circuit 18. For another system, also depending on the level of the output of the difference-hold circuit 18, the speed of rotation of the motor 22 is adjusted. In such a manner, the helicoid controlling loop of the motor 22 climbs the hill of the characteristic of the focus voltage produced from the detector 16, finally reaching the peak of this hill. In other words, the motor 22 is controlled so that it stops when the focus voltage reaches a maximum at the in-focus position A.
But, this arrangement has the following problem. That is, because the system operates in such a manner that when the in-focus point is reached, the motor 22 stops, whereby the focus voltage produced from the detector 16 becomes constant, if, in this state, the object to be photographed is altered by panning, or hand-shake, or like situation is encountered, the object image on the image pickup surface of the camera circuit 12 changes to decrease the high-frequency component. Therefore, the level of the focus voltage produced from the detector 16 lowers as shown in FIG. 3. As far as the control loop of FIG. 1 is concerned, the object to be photographed or the camera is then taken as having moved, and the motor 22 is driven again so as to maximize the focus voltage. Since, in this situation, the distance between the object and this video camera actually remains unchanged, the excursion of the lens 10 stops at the same position as before the re-energization of the motor, and the focus voltage comes to rest at the same level as before the re-energization of the motor as shown in FIG. 3. The occurrence of such an operation results in the production of a large defocus for a while. Hence the shot picture becomes very unnatural and uncomfortable to view.
To solve this, a method has been proposed (in Japanese Laid-Open Patent Application No. Sho 57-208520) that when the object image on the image pickup plane suddenly varies or changes, energization of the motor 22 is prohibited for a prescribed time. For this purpose, a circuit is provided for detecting the above-described variation or change of the object image. During the time when the object image on the image pickup plane of the image pickup element changes, the connection between the hill climb computer portion (for example, the difference-hold circuit 18) and the motor drive portion (for example, the motor drive circuit 20) is cut off by means of a gate circuit to interrupt the hill climbing operation for a time. But, there is an alternative problem that the relationship between the period of varying of the object image or the panning period and the time point at which the motor 22 is to be energized again is uncertain.
Meanwhile, on consideration of another case where, as the automatic focusing on the object to be photographed goes on, when it happens that object no longer stands still, but starts moving to vary the object image on the image pickup plane, according to the above-described conventional example of FIG. 1, variation of the object causes the focus voltage to vary. Thus, the difference-hold circuit output shown in FIG. 2 cannot be obtained. As a result, an erroneous operation takes place in the hill climb process. Hence there is some possibility of occurrence of an accidental large defocus.
Also, if the hill climbing operation is interrupted each time the object to be photographed starts to move, there is a drawback that it takes a very long time from the stoppage of the object to the attainment of the sharp focus on the object.
Therefore, a device capable of correcting the characteristic of the automatic focusing operation in accordance with variation of the object to be photographed during the time when automatic focusing is performed is desired.
Also, further, in general, according to such a video camera, there is the following drawback too. That is, when focus adjustment is carried out, it is usual that the light receiving plane of the solid state image pickup element is given an area for automatic focusing of a prescribed size (hereinafter called "distance measuring" area), and the automatic focusing operation is performed by the signal from this area. But, the lens used in the video camera is a zoom lens whose range is usually 4-10. For the object at a short distance, when in wide angle photography, the object image is present in an almost entire area of the light receiving plane. For a distant object, on the other hand, when in wide angle photography, the object image is present only in a small portion of the light receiving plane. Therefore, the unchangeability of the size of the distance measuring area leads to a possibility of occurrence of a faulty focusing operation. That is, because, in the case of the short distance with the wide angle photography, the object image oversteps the distance measuring area, the object information becomes insufficient. Conversely in the case of the long distance with the wide angle photography, unnecessary information mixes in.
To solve such problems, a method has been proposed for enlarging or reducing the distance measuring area in response to the lens state with reference to the central portion of the light receiving plane.
But, by this prior known method, in a case when the object image falls outside the distance measuring area as the object is moving, as shown in FIG. 4, or in another case when the object image is moved beyond the distance measuring area by panning, as shown in FIG. 5, it results that the automatic focusing is controlled by using a portion of the area which does not contain the image of the object intended to be photographed. A problem arises in that focusing cannot be effected reliably on an object aimed at.